The phraze that the character FES(FEZ) off of the show 'That '70's show' says when one of the other characters makes him mad or frustrated.

FES Good Day!!!
Donna: But FES...?
FES: I said GOOD DAY!!!(runs out the door)

Kelso: FES you're just mad that your chickens and lizards don't fit into my pinball machine.
FES: I will not dignify that with a responce... for i cannot think of one, but when i do... good day!
Kelso: But FES...
FES: I said kiss my a$$

The day you accomplish something you've been meaning to do for a long time. It usually involves doing something bad ass and having people give you compliments all day, such as "that was so cool" or "you are god".

Valerie came up and started bugging me again so I fucked her shit up; it was a good day.

something said by one Brit to another in an attempt at cordiality when in actuality both parties despise the other (which can be attributed to irritablility caused by bad weather and overly high socks.)